If you are coming close to Dalvik's memory limit (and you obviously do...) especially native memory reservations start to act crazy. As said, i had similar issues when loading images (which seems to use some native memory internally). Behavior was unpredictable...sometimes it worked, sometimes it failed. Googling the issue resulted in similar problems all over the place but no real solution... The compact()-method in the memory helper is the result of this...

However, i can still bring memory usage down by 0-1MB and i'll add that to the next version. In addition, you could prevent textures used for blitting only from using mipmapping, if you haven't already done that.